English

• 0.   Introduction

  Metal - organic complexes are an interesting and useful class of compound. During the past decades, the synthesis and characterization of metal-organic com-plexes is rapidly developing in chemical science^[1-3]. The reason is that their interesting structures and topol-ogy are diverse on the one hand, on the other hand, their potential applications are tempting, such as gas storage and separation, biomedical applications, envi-ronmental pollution control and catalysis^[4-7]. Metal-organic complexes as a rapidly developing class of organic-inorganic hybrid materials are constructed from metal ion/cluster nodes and functional organic ligands through coordination bonds. According to the composition, the certain features of ligands are crucial, such as flexibility, functionality and versatile binding modes^[8-10]. In order to research the biomedical applica-tions of complexes, the biocompatibility must be con-sidered as important as the function. Thus, more and more natural biomolecules as ligands were considered to be used. Nucleobase is one of the first such ligands used to synthesize complexes in the form of crystals successfully. In 2009, zinc-adeninate framework bio-MOF-1 with crystal structure was reported by Rosi group^[11]. After that more and more complexes as crystals were synthesized based on biomolecules, such as bio-MOF-100 and medi-MOF-1^[12-15]. However, compared to other types of metal - organic complexes, the number of metal-biomolecule complexes is very lit-tle. Thus, it is necessary to research the method using biomolecules as ligands to synthesize crystalline metal-organic complexes.

  In this work, we choose orotic acid (H₃Or) as ligand. H₃Or (vitamin B13) as a uracil carboxylic acid is one simple natural biomolecule ligand, and it plays a key role in the biosynthesis of pyrimidine bases of nucleic acids. Therefore, H₃Or has biological activi-ty^[16-20]. On the other side, comparing with ligands only containing carboxylate or pyridyl group, as we all know, the reports on pyridyl - carboxylate ligands are also less to now. The carboxylate group can have vari-able coordination modes and strong coordination abili-ties, while the pyridyl-containing ligands have been demonstrated to be powerful in construction of com-plexes with novel structures and topologies^[21-23]. From the analysis of structure, orotic acid as ligand has the advantages of both carboxylic acid and pyridine (Scheme 1). However, because of the structure with low symmetry, it is difficult to synthesize ideal crystals using H₃Or as ligand^[24-27]. In this work, we designed and synthesized three new complexes, namely {[Cu (HOr)₂] ·2NH₂(CH₃)₂}_n (R - 1), [Co₂(HOr)₂(bipy) (H₂O)₆] · 2H₂O (R - 2) and {[Ni₂(HOr)₂(1, 3-dpp)₂(H₂O)₂] [Ni(HOr) (1, 3-dpp)(H₂O)]₂·(1, 3-dpp)·2H₂O}_n (R- 3) (H₃Or=orotic acid, bipy=4, 4' - bipyridine and 1, 3 - dpp=1, 3 - di(4 - pyridyl)propane). R-1 is mononuclear complex with 2D sheet structure, and R-2 is a simple monomolecular complex. R-3 is a complicated structure containing 2D layers and 1D chains, which looks like sandwich. In the complexes, intermolecular hydrogen bonding inter-actions exist, which result in 3D supramolecular frame-works, respectively. In addition, the thermal stability and infrared spectra property of the complexes were investigated.

  Scheme 1

  

  Scheme 1.  Structure of orotic acid

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  1.   Experimental

  1.1   Materials and methods

  All commercially available chemicals and sol-vents are of reagent grade and were used as received without further purification. Elemental analysis for C, H and N was performed on a Perkin-Elmer 240 C Ele-mental Analyzer. FT-IR spectra was recorded in a range of 400~4 000 cm^-1 on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Thermogravimet-ric analysis (TGA) was performed on a simultaneous SDT 2960 thermal analyzer under nitrogen with a heat-ing rate of 10 ℃·min^-1.

  1.2   Synthesis of complexes R-1, R-2 and R-3

  1.2.1   Synthesis of R-1

  H₃Or (15.6 mg, 0.1 mmol) and biphenyl - 4, 4' - dicarboxylic acid (24.2 mg, 0.1 mmol) were added in DMF (8 mL) and stirred. After two hours, CuSO₄·5H ₂O (25.0 mg, 0.1 mmol) was added. Then, the mixture was sealed in a 20 mL Teflon - lined steel bomb for half an hour and heated at 90 ℃ for three days. Blue block sin-gle crystals of R -1 were obtained in 35% yield. Anal. Calcd. for C₁₄H₂₀N₆O₈Cu(%): C, 36.25; H, 4.35; N, 18.12. Found(%): C, 36.61; H, 4.16; N, 18.51. IR (KBr pellet, cm^-1): 3 120(s), 1 651(s), 1 480(m), 1 401(s), 1 342(m), 1 220(s), 1 029(m), 947(m), 847(w), 791(s), 685(w), 592(w) (Fig.S3a).

  1.2.2   Synthesis of R-2

  H₃Or (1.56 g, 10 mmol) and KOH (1.68 g, 30 mmol) were stirred in 100 mL H₂O for four hours, and this solution was called S-1. 1.0 mL S-1, 1.0 mL Co(OAc)₂·4H ₂O (0.1 mol·L^-1) aqueous solution and 4, 4' - bipyridine (15.6 mg, 0.1 mmol) were added in 8.0 mL H₂O and stirred for two hours. The resultant solu-tion was sealed in a 20 mL Teflon-lined stainless -steel container and heated at 90 ℃ for 3 days. Orange block single crystals of R-2 were obtained in 65% yield. Anal. Calcd. for C₂₀H₂₈N₆O₁₆Co₂(%): C, 33.07; H, 3.89; N, 11.57. Found(%): C, 33.21; H, 3.56; N, 11.72. IR (KBr pellet, cm^-1): 3 364(s), 1 623(s), 1 480(m), 1 384 (s), 1 321(m), 1 217(m), 1 015(m), 805(s), 633(m), 576 (w) (Fig.S3b).

  1.2.3   Synthesis of R-3

  1.0 mL S-1, 1.0 mL Ni(OAc)₂·4H₂O (0.1 mol·L^-1) aqueous solution and 1, 3 - di(4 - pyridyl)propane (19.8 mg, 0.1 mmol) were added in 8.0 mL H ₂O and stirred for two hours. The resultant solution was sealed in a 20 mL Teflon-lined stainless - steel container and heated at 150 ℃ for 3 days. Green block single crystals of R - 3 were obtained in 52% yield. Anal. Calcd. for C₈₅H ₉₀N₁₈O₂₂Ni₄(%): C, 52.34; H, 4.65; N, 12.93. Found (%): C, 52.65; H, 4.56; N, 12.51. IR (KBr pellet, cm^-1): 3 404(s), 1 620(s), 1 469(m), 1 378(s), 1 328(w), 1 220 (s), 1 071(m), 948(m), 853(w), 795(s), 674(w), 615(w) (Fig.S3c).

  1.3   Crystallographic data collection and refinement

  Diffraction data for R-1, R-2 and R-3 were col-lected on a Bruker Smart Apex Ⅱ CCD with graphite monochromated Mo Kα radiation source (λ=0.071 073 nm) in φ- ω scan mode. The diffraction data were inte-grated by using SAINT program^[28], which was also used for the intensity corrections for the Lorentz and polar-ization effects. Semi - empirical absorption corrections were applied using SADABS program^[29]. All the struc-tures were solved by direct methods using SHELXS - 97^[30] and the non - hydrogen atoms were refined on F² by full-matrix least-squares procedures with SHELXL-2018^[31]. The final formulas were calculated from TGA and elemental analysis. Hydrogen atoms except those of water molecules were generated geometrically and refined isotropically using the riding model. The hydro-gen atoms of coordinated water molecules were found directly. The details of the crystal parameters, data col-lection and refinements for R-1, R-2 and R-3 are sum-marized in Table 1. Selected bond lengths and angles for R-1, R-2 and R-3 are listed in Table S1 (Support-ing information). The parameters of hydrogen bonds for R-1, R-2 and R-3 are listed in Table S2.

  Table 1

  

  Table 1.  Crystal data and structure refinements for complexes R-1, R-2 and R-3

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  Complex	R-1	R-2	R-3
  Formula	C14H20N6O8Cu	C20H28N6O16Co2	C85H90N18O22Ni4
  Formula weight	463.90	726.34	1 950.58
  Crystal system	Monoclinic	Triclinic	Triclinic
  Space group	P21/c	P1	P1
  a / nm	1.003 94(18)	0.605 9(5)	1.078 5(3)
  b / nm	0.977 15(18)	1.069 0(5)	1.486 9(5)
  c / nm	0.901 68(16)	1.168 9(5)	2.802 0(8)
  α/(°)		66.807(5)	82.737(5)
  β/(°)	92.401(2)	83.841(5)	86.264(5)
  γ/(°)		85.606(5)	82.435(5)
  V / nm3	0.883 8(3)	0.691 4(7)	4.413(2)
  Z	2	1	2
  Dc / (g·cm-3)	1.743	1.744	1.468
  μ / mm-1	1.297	1.288	0.924
  F(000)	478	372	2 028
  Unique reflection	1 775	2 714	10 426
  Observed reflection [I > 2σ(I)]	2 002	3 032	16 512
  Parameter	135	199	1168
  GOF	1.004	1.075	1.036
  Final R indices [I > 2σ(I)]a, b	R1=0.037 1, wR2=0.122 3	R1=0.039 9, wR2=0.133 1	R1=0.069 8, wR2=0.204 2
  R indices (all data)	R1=0.044 1, wR2=0.142 1	R1=0.044 7, wR2=0.140 6	R1=0.108 7, wR2=0.227 0
  Largest difference peak and hole / (e·nm-3)	760 and -524	786 and -664	2 059 and -638
  aR1=Σ||Fo|-|Fc||/Σ|Fo|; b wR2=[Σw(Fo2-Fc2)2]/Σw(Fo2)2]1/2.

  CCDC: 1983479, R-1; 1983481, R-2; 1983482, R-3.

  2.   Result and discussion

  2.1   Crystal structure of R-1, R-2 and R-3

  2.1.1   Crystal structure of R-1

  The crystal structure of R-1 was determined by single-crystal X-ray diffraction analysis. R-1 is a mono-nuclear complex crystallizing in P2₁/c space group. The asymmetric unit of R-1 consists of one Cu(Ⅱ) ion, one HOr^2- ligand, and free one dimethylamine ion (Fig. S1). Each Cu(Ⅱ) is coordinated by two chelated sym-metry related HOr^2- ligands through carboxylate oxy-gen O1 and N1 in the equatorial plane resulting in [Cu(HOr)₂] secondary building unit (SBU). In the trans axial positions, there are two other oxygen atoms (O2ⁱⁱ and O2ⁱⁱⁱ) from other [Cu(HOr) ₂] SBUs linked with Cu(Ⅱ), and the distance between Cu(Ⅱ) and O2ⁱⁱ or O2ⁱⁱⁱ is 0.273 3 nm measured by Mercury program, which results in a 4+2 coordination environment around each Cu(Ⅱ) (Fig. 1a). The coordination environment is similar to the reported complex {[Cu(H₂iso)₂] ·H ₂ O}_n (H₃iso= isoorotic acid) ^[15]. Such [Cu(HOr)₂] SBUs adjacent to each other link to form a 2D sheet parallel to bc plane (Fig. 1b). Adjacent 2D sheets through H-bonding inter-action connect to form a 3D framework (Fig. 1c) and the holes of the framework are occupied by dimethylamine molecules (Fig. 1d).

  Figure 1

  

  Figure 1.  (a) Coordination environment around Cu(Ⅱ) center(Hydrogen atoms have been omitted for clarity; Symmetry codes: ⁱ 1-x, 1-y, 1-z; ⁱⁱ x, 1/2-y, 1/2+z; ⁱⁱⁱ 1-x, 1/2+y, 1/2-z); (b) 2D sheet based on [Cu(HOr)₂] SBUs parallel to bc plane; (c) H-bonding between 2D sheet along a and c axes showed by the yellow dashed line; (d) 3D structure of R-1

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  2.1.2   Crystal structure of R-2

  R-2 as a monomolecular complex crystallizes in the triclinic P1 space group by single crystal X-ray structural analysis. The asymmetric unit of R-2 con-sists of one Co(Ⅱ)ion, one HOr^2- ligand, half 4, 4'-bipy ligand and three coordinated water molecules. The coordination number of Co(Ⅱ)ion is six. Co(Ⅱ)ion is the center with distorted octahedron coordination geome-try, in which the equatorial positions are occupied by N1, N3, O2 and O3W, and the axial positions are occu-pied by coordinated water molecules (Fig. 2a). Two [Co(HOr) (H₂O)₃] units link together by 4, 4'-bipy to form [Co₂(HOr)₂(4, 4'-bipy) (H₂O)₆] dimer. Between [Co₂ (HOr)₂(4, 4'-bipy) (H₂O)₆] dimers, there are hydro-gen bonding interactions. Such dimers connect together to form a 1D chain by N2- H2…O3 (Fig. 2b and Table S2). Though intermolecular hydrogen bonding interac-tion from two axial positions O1W and O2W, the chains link together to form irregular layer structure (Fig. 2c and Table S2). The 2D layers are further extended into a 3D structure though O2W-H2O2…O2 interaction (Fig. 2d and Table S2).

  Figure 2

  

  Figure 2.  (a) Coordination environment of Co(Ⅱ)ion in R-2 with 50% thermal ellipsoid probability (Hydrogen atoms have been omitted for clarity; Symmetry code: ⁱ -x, - 1-y, 2 -z); (b) 1D chain formed by N2-H2…O3 intermolecular hydrogen bonding interaction shown by dashed black line; (c) Intermolecular hydrogen bonding interaction at two axial positions (purple: O2W-H1O2…O1, blue: O1W-H2O1…O4); (d) 3D structure of R-2 with hydrogen bonds indicated by dashed lines (black: N2-H2…O3, purple: O2W-H1O2…O1, green: O2W-H2O2…O2, blue: O1W-H2O1…O4)

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  2.1.3   Crystal structure of R-3

  As far as we know, R-3 has the most complex structure based on H₃Or as ligand in the reports until now^[15-27]. Crystallographic analysis indicates that R-3 crystallizes in the triclinic space group P1 with compli-cated sandwich structure. In the structure of R-3, except free 1, 3-dpp and water molecules, there are six different coordination modes of Ni(Ⅱ).

  Ni1 is coordinated with distorted octahedron coor-dination geometry, in which the equatorial positions are occupied by O1A, O6A, N3A and N6A, and the axi-al positions are occupied by N1A and N8A (Fig. 3a). Among them, O1A and N3A come from one HOr^2-, while O6A and N6A come from another HOr^2-. For Ni2, it is coordinated with O2A, O5A, O1W, O2W, N2A and N7A. Among them, O2A and O5A come from different HOr^2-, while N2A and N7A come from differ-ent 1, 3 - dpp molecules. Thus, the carboxyl groups of HOr^2- bridge adjacent Ni1 and Ni2 to form a chain along a axis (Fig. 3b). Along c axis, the adjacent chains connect the auxiliary ligand of 1, 3-dpp to form a layer (Fig. 3b).

  Figure 3

  

  Figure 3.  (a) Coordination environment of Ni(Ⅱ) in R-3 with ellipsoids drawn at 50% probability level (The hydrogen atoms and free 1, 3-dpp and water molecules are omitted for clarity; Symmetry codes: ⁱ 1+x, y, z; ⁱⁱ 1-x, 1-y, 1-z; ⁱⁱⁱ 1-x, 1-y, -z; ^iv 1-x, -y, 1-z; ^v 2-x, -y, 1 -z; ^vi -x, -y, -z; ^vii 1-x, -y, -z); (b) Chains based on Ni1, Ni2 and bridging HOr^2- ligands linked together by 1, 3-dpp to form 2D layer in R-3; (c) 3D sandwich structure of R-3 containing the chains formed by Ni3-Ni6 and the layers formed by Ni1 and Ni2 (the layers formed by Ni1 and Ni2 are shown in green; the chains formed by Ni3 and Ni4 are shown in yellow; the chains formed by Ni5 and Ni6 are shown in purple)

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  Except Ni1 and Ni2, the coordination numbers of Ni3, Ni4, Ni5 and Ni6 are also six. Besides, the coordi-nated mode of Ni3 is similar to Ni5, and Ni4 is similar to Ni6. For Ni3, it is coordinated by four N atoms and two O atoms with distorted octahedron coordination geometry. Besides, two O1B atoms and two N1B atoms occupy the equatorial positions and two N4B from dif-ferent 1, 3-dpp molecules occupy the axial positions for Ni3 (Fig. 3a). The carboxyl group of HOr^2- ligand and N3B and N4B of 1, 3-dpp bridge adjacent Ni3 and Ni4. The remainder positions of Ni4 are coordinated with water molecules. Such adjacent Ni3 and Ni4 are linked by bridging the carboxyl groups and 1, 3-dpp molecules to form a chain along a axis, which is same to Ni5 and Ni6 (Fig. 3c). Between the adjacent layers based on Ni1 and Ni2, the chains based on Ni3~Ni6 fill. Thus, the sandwich structure is formed (Fig. 3c). Interestingly, in the sandwich structure, intermolecular hydrogen bonding interaction also exists between the layers and the chains (Fig.S2 and Table S2).

  2.2   Synthesis of the complexes and comparison of the structures

  Comparing with other organic ligands, the reports on biomolecules are less. In this work, three metal-organic complexes were synthesized and characterized based on orotic acid (H₃Or), a simple natural biomole-cule. In the complexes, although all H₃Or molecules loss two protons and becomes HOr^2-, there are two coor-dination modes of HOr^2- ligand (Fig. 4). In the struc-tures of R-1 and R-2, all HOr^2- ligands adopt chelate mode (mode Ⅰ), in which one oxygen atom of the car-boxylate group and one adjacent nitrogen atom coordi-nate one metal ion, and other oxygen and nitrogen atoms are not involved in coordination. This chelate mode is the main reason leading to the low dimensional structure of R-1 and R-2. Similarly, the complicated sandwich structure of R-3 is also inseparable from the coordination mode of HOr^2- ligand. In R-3, all HOr^2-ligands adopt another coordination mode (mode Ⅱ). In mode Ⅱ, one oxygen atom of the carboxylate group with chelate mode links one metal ion as mode Ⅰ, and the other oxygen atom of the carboxylate group with bridge mode links another metal ion. Thus, the struc-ture of R-3 is more complex.

  Figure 4

  

  Figure 4.  Two coordination modes of HOr^2- ligand in R-1, R-2 and R-3

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  In addition, the deprotonation of carboxylate group for H₃Or was confirmed as described above by crystal structural analyses as well as by IR spectra. In IR spectra, no obvious bands between 1 680 and 1 760 cm^-1 were found, which are originated from the carbox-ylic group (- COOH) (Fig.S3). Although biphenyl-4, 4'-dicarboxylic acid does not exist in the final structures of R-1, it should be pointed out that it may play impor-tant role in the formation of R-1. Because without addi-tion of biphenyl-4, 4' -dicarboxylic acid in the synthetic reactions, no crystals were obtained.

  2.3   Thermogravimetric analyses

  The thermal stability of complexes is important for their application research, so thermogravimetric analy-ses (TGA) of R-1, R-2 and R-3 were measured. The thermal stability was examined by TGA in the N₂ atmo-sphere from 30 to 800 ℃ (Fig. 5). The result indicates that R-1 showed a weight loss of 19.2% before 200 ℃, which corresponds to the release of dimethylamine (Calcd. 19.8%). Above 200 ℃, R-1 began to decom-pose. For R-2, the first weight loss of 5.1% from 30 to 170 ℃ is in accordance with the loss of free water mole-cules (Calcd. 5.0%). The further weight loss was observed at about 350 ℃, owing to the decomposition of R-2. For R-3, before 180 ℃ the weight loss was 12.8% and it results from the loss of free water and 1, 3-dpp molecules (Calcd. 12.1%).

  Figure 5

  

  Figure 5.  Thermogravimetric curves of R-1, R-2 and R-3

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  3.   Conclusions

  In summary, three metal-organic complexes based on orotic acid were synthesized and characterized. The orotic acid losses two protons and there are two coordi-nation modes in the complexes. R-1 is 2D sheet struc-ture, R-2 is a simple monomolecular complex and R-3 is a complex sandwich structure containing layers and chains. Three-dimensional frameworks of the complex-es can be formed by intermolecular hydrogen bonding interactions. Furthermore, the thermal stability and infrared spectra property of the complexes were researched.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Scheme 1  Structure of orotic acid

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  Figure 1  (a) Coordination environment around Cu(Ⅱ) center(Hydrogen atoms have been omitted for clarity; Symmetry codes: ⁱ 1-x, 1-y, 1-z; ⁱⁱ x, 1/2-y, 1/2+z; ⁱⁱⁱ 1-x, 1/2+y, 1/2-z); (b) 2D sheet based on [Cu(HOr)₂] SBUs parallel to bc plane; (c) H-bonding between 2D sheet along a and c axes showed by the yellow dashed line; (d) 3D structure of R-1

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  Figure 2  (a) Coordination environment of Co(Ⅱ)ion in R-2 with 50% thermal ellipsoid probability (Hydrogen atoms have been omitted for clarity; Symmetry code: ⁱ -x, - 1-y, 2 -z); (b) 1D chain formed by N2-H2…O3 intermolecular hydrogen bonding interaction shown by dashed black line; (c) Intermolecular hydrogen bonding interaction at two axial positions (purple: O2W-H1O2…O1, blue: O1W-H2O1…O4); (d) 3D structure of R-2 with hydrogen bonds indicated by dashed lines (black: N2-H2…O3, purple: O2W-H1O2…O1, green: O2W-H2O2…O2, blue: O1W-H2O1…O4)

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  Figure 3  (a) Coordination environment of Ni(Ⅱ) in R-3 with ellipsoids drawn at 50% probability level (The hydrogen atoms and free 1, 3-dpp and water molecules are omitted for clarity; Symmetry codes: ⁱ 1+x, y, z; ⁱⁱ 1-x, 1-y, 1-z; ⁱⁱⁱ 1-x, 1-y, -z; ^iv 1-x, -y, 1-z; ^v 2-x, -y, 1 -z; ^vi -x, -y, -z; ^vii 1-x, -y, -z); (b) Chains based on Ni1, Ni2 and bridging HOr^2- ligands linked together by 1, 3-dpp to form 2D layer in R-3; (c) 3D sandwich structure of R-3 containing the chains formed by Ni3-Ni6 and the layers formed by Ni1 and Ni2 (the layers formed by Ni1 and Ni2 are shown in green; the chains formed by Ni3 and Ni4 are shown in yellow; the chains formed by Ni5 and Ni6 are shown in purple)

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  Figure 4  Two coordination modes of HOr^2- ligand in R-1, R-2 and R-3

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  Figure 5  Thermogravimetric curves of R-1, R-2 and R-3

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  Table 1.  Crystal data and structure refinements for complexes R-1, R-2 and R-3

  Complex	R-1	R-2	R-3
  Formula	C14H20N6O8Cu	C20H28N6O16Co2	C85H90N18O22Ni4
  Formula weight	463.90	726.34	1 950.58
  Crystal system	Monoclinic	Triclinic	Triclinic
  Space group	P21/c	P1	P1
  a / nm	1.003 94(18)	0.605 9(5)	1.078 5(3)
  b / nm	0.977 15(18)	1.069 0(5)	1.486 9(5)
  c / nm	0.901 68(16)	1.168 9(5)	2.802 0(8)
  α/(°)		66.807(5)	82.737(5)
  β/(°)	92.401(2)	83.841(5)	86.264(5)
  γ/(°)		85.606(5)	82.435(5)
  V / nm3	0.883 8(3)	0.691 4(7)	4.413(2)
  Z	2	1	2
  Dc / (g·cm-3)	1.743	1.744	1.468
  μ / mm-1	1.297	1.288	0.924
  F(000)	478	372	2 028
  Unique reflection	1 775	2 714	10 426
  Observed reflection [I > 2σ(I)]	2 002	3 032	16 512
  Parameter	135	199	1168
  GOF	1.004	1.075	1.036
  Final R indices [I > 2σ(I)]a, b	R1=0.037 1, wR2=0.122 3	R1=0.039 9, wR2=0.133 1	R1=0.069 8, wR2=0.204 2
  R indices (all data)	R1=0.044 1, wR2=0.142 1	R1=0.044 7, wR2=0.140 6	R1=0.108 7, wR2=0.227 0
  Largest difference peak and hole / (e·nm-3)	760 and -524	786 and -664	2 059 and -638
  aR1=Σ||Fo|-|Fc||/Σ|Fo|; b wR2=[Σw(Fo2-Fc2)2]/Σw(Fo2)2]1/2.

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